I. Field
The present invention relates to data communication. More particularly, the present invention relates to re-formatting variable-rate vocoder frames for intersystem transmissions.
II. Description of the Related Art
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for remote subscribers. As used herein, the term “cellular” system encompasses systems using either cellular or personal communications services (PCS) frequencies. Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems are promulgated by the Telecommunication Industry Association (TIA) and other well-known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and incorporated by reference herein. An exemplary system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in the draft versions of IS-2000 and has been approved by the TIA. Another CDMA standard is the W-CDMA standard, as embodied in 3rd Generation Partnership Project “3GPP”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.
Each standard defines how various types of information are processed for transmission. In a typical communication system, an encoder generates a stream of information bits representing voice or data traffic. This stream of bits is subdivided and grouped, concatenated with various control bits, and packed into a suitable format for transmission. Voice and data traffic can be transmitted in various formats according to the appropriate communication standard, such as, e.g., frames, packets, and subpackets. For the purpose of illustrative ease, the term “frame” will be used herein to describe the transmission format in which traffic is carried. However, the term “frame” will also be used herein to describe the output of a speech coder. The definition of the word will depend upon the context in which the word is used
A speech coder is a device that extracts parameters relating to a model of human speech generation and then uses these parameters to compress the speech for transmissions. Speech coders typically comprise an encoder and a decoder. A speech coder divides the incoming speech signal into blocks of time, or analysis frames. The encoder analyzes the incoming speech frame to extract certain relevant parameters, and then quantizes the parameters into binary representation. The binary representation is packed into transmission frames and is transmitted over a communication channel to a receiver with a decoder. The decoder processes the transmission frames, unquantizes them to produce the parameters, and resynthesizes the speech frames using the unquantized parameters. Speech coders are also referred to as voice coders, or “vocoders,” and the terms will be used interchangeably herein.
The function of the speech coder is to compress the digitized speech signal into a low-bit-rate signal by removing all of the natural redundancies that are inherent in speech. The digital compression is achieved by representing the input speech frame with a set of parameters and employing quantization to represent the parameters with a set of bits. If the input speech frame has a number of bits Ni and the output frame produced by the speech coder has a number of bits No, then the compression factor achieved by the speech coder is Cr=Ni/No. The challenge is to retain the high voice quality of the decoded speech while achieving a target compression factor. The performance of a speech coder depends on how well the speech model, or the combination of the analysis and synthesis process described above, performs, and how well the parameter quantization process is performed at the target bit rate of No bits per frame. Thus, the goal of the speech model is to capture the essence of the speech signal, or the target voice quality, with a small set of parameters for each frame.
Different types of speech coders are deployed in the various existing wireless communication systems, often using quite dissimilar speech compression techniques. Moreover, the transmission frame formats and processing that are defined by one particular standard is most likely different from those of other standards. For example, CDMA standards support the use of variable-rate vocoder frames in a spread spectrum environment while GSM standards support the use of fixed-rate vocoder frames and multi-rate vocoder frames. Similarly, Universal Mobile Telecommunications Systems (UMTS) standards also support fixed-rate and multi-rate vocoders, but not variable-rate vocoders. For compatibility and interoperability between these communication systems, it is highly desirable to enable the support of variable-rate vocoder frames within GSM and UMTS systems, and the support of non-variable rate vocoder frames within CDMA systems. The support of multi-rate vocoder frames within CDMA standards is addressed in co-pending U.S. patent application Ser. No. 09/755,843, entitled, “METHOD AND APPARATUS FOR SUPPORTING ADAPTIVE MULTI-RATE (AMR) DATA IN A CDMA COMMUNICATION SYSTEM,” which is assigned to the assignee of the present invention and incorporated by reference herein. However, there is still a present need to support the transmission of variable-rate vocoder frames within heretofore non-compatible systems.